1. Field of the Invention
The present disclosure is generally related to communication systems, and, more particularly, is related to wireless communication systems and methods
2. Related Art
Wireless communication systems are widely deployed to provide various types of communication, such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division multiplex (OFDM), or some other multiplexing techniques. Continual demand for increased data rates has resulted in the advancement of communications system technology, such as the use of multiple antennas in a single device having transmitter and/or receiver functionality. However, in systems utilizing multiple-antenna devices, there is a need to consider legacy receivers (e.g., single-input, single output (SISO), OFDM receivers) and the design challenges concomitant with implementing transmitters with multiple antennas in an environment that still uses legacy receivers.
FIG. 1 is a block diagram that illustrates such a mixed antenna transmitter device/legacy environment. As shown, an 802.11 compliant (e.g., 802.11a, 802.11g) multi-antenna (MA) transmitter device 102 is shown in communication with a SISO 802.11 receiver device 112 (herein simply SISO receiver device 112). The MA transmitter device 102 comprises antennas 104 and 106. The SISO receiver device 112 comprises an antenna 110. The same signals transmitted from antennas 104 and 106 may follow direct paths and multi-paths to antenna 110. Normally, multiple-path signals (e.g., resulting from reflections off of objects in the environment or from multiple antennas) can result in either partial or complete constructive interference or destructive interference of the same signals in a SISO receiver. One mechanism to avoid or mitigate completely destructive interference from multi-path signals is to equip one of the antennas 104 and 106 with a time delay element 108 (or provide a delay through a baseband processing section of the MA transmitter device 102). From the perspective of the SISO receiver device 112, the time delay element 108 appears to cause the signal sent from antenna 106 to reach antenna 110 later than the same signal sent from antenna 104. This technique is often referred to as linear delay diversity, and it's impact on performance of the SISO receiver device 112 is described below.
FIG. 2 is block diagram that describes an exemplary OFDM packet structure 200 used in the transmission of information between the MA transmitter device 102 (FIG. 1) and the SISO receiver device 112 (FIG. 1). Additional information about the packet structure can be found in 802.11 standards. The packet structure 200 is generated in a baseband processing section (e.g., in or in cooperation with an inverse fast Fourier transform (IFFT) operation) of the MA transmitter device 102, and comprises several sections. Sections A and B are comprised of short training symbols (STS). Section A is used by a communication system to provide signal detection, automatic gain control (AGC), and diversity selection functionality. Section B is used by a communication system to provide coarse frequency offset estimation and timing synchronization. Section C, sometimes referred to as a long training symbol (LTS), is used by a communication system to provide channel and fine frequency offset estimation. Section D is referred to as the signal field or header, and contains data rate and packet length information. Sections E and F are OFDM symbols. Sections D, E, and F provide rate length, service and data, and data, respectively. Section F is used as an exemplary OFDM symbol for the discussion below, and will be referred to as an OFDM symbol 300.
FIG. 3A is a block diagram that illustrates exemplary OFDM symbols 300a, 300b, and 300c (herein, simply referenced as OFDM symbol 300) shown in a time-domain perspective. Each OFDM symbol 300 comprises a guard interval (GI) 302, which provides a buffer between symbols 300. That is, multipath is equivalent to echoes, and the guard intervals 302 comprise a zone built into an OFDM symbol 300 into which echoes from a preceding OFDM symbol are positioned. Such echoes cause inter-symbol interference. The guard intervals 302 are inserted in the OFDM symbol 300 in the baseband processor portion of the MA transmitter device 102 (FIG. 1), and are used to capture unwanted multipath-induced inter-symbol interference at the SISO receiver device 112 (FIG. 1). In an 802.11 implementation, each symbol 300 is approximately 4 microseconds (μsec) in duration. For instance, of the 80 samples in an OFDM symbol waveform, the guard interval 302 consumes 16 samples (the data portion comprising 64 samples). Fast Fourier transforms (FFT) applied at the SISO receiver device 112 (e.g., 64-point FFT) occur (or are placed) in the 3.2 (μsec) interval of each OFDM symbol 300. That is, the SISO receiver device 112 discards the 0.8 μsec duration guard interval 302 on each received 4 μsec OFDM symbol, leaving 3.2 μsec of signal (which comprises all of the information of the OFDM symbol) to FFT. No echoes are included in the 3.2 μsec interval (inter-symbol interference free). Thus, reference to “placing” or “to place” or “placement” of the FFT is understood to comprise a process or method of extracting the correct 3.2 μsec from each 4 μsec interval for implementing the FFT.
FIG. 3B is a sub-carrier level view of the OFDM symbol 300 of FIG. 3A shown in a frequency domain perspective, and is helpful to consider in understanding how the guard intervals 302 (FIG. 3A) are created. An OFDM symbol 300 comprises a plurality of frequency subcarriers or tones 304 (e.g., 52 data subcarriers in an 802.11a or 802.11g OFDM symbol, although only a portion of the 52 are shown) distributed in both frequency directions from a center or direct current (DC) frequency. The subcarriers or tones 304 spin at progressively increasing rotations in an in-phase (I)/quadrature (Q) plane from center frequency. Assuming no guard interval insertion, the subcarrier located at frequency position 306 spins counter-clockwise (e.g., in a direction of increasing positive angles off of the x-axis) in an I/Q plane one cycle over one-64 sample period of the symbol 300. The subcarrier located at frequency position 308 spins clockwise (e.g., in a direction of negative angles off of the x-axis) in an I/Q plane one cycle over the 64 samples. The subcarrier located at frequency position 310 spins counter-clockwise two cycles over the 64 samples, and the subcarrier located at frequency position 312 spins clockwise two cycles over the 64 samples, and so on. Thus, with the guard intervals left out, what remains is a continuous waveform that repeats itself every 64-samples. The guard intervals 302 are a time domain extension of the subcarriers 64 time samples, pre-spinning an additional 16 time domain samples. That is, subcarrier time samples are allowed to “pre-spin” out 16 samples, resulting in guard intervals being created as cyclic extensions of the spin of the subcarrier time samples. Thus, each subcarrier is allowed to spin approximately 25% more than the subcarrier spin without the guard intervals 302.
In light of the aforementioned background, and with continued reference to FIGS. 1-3B, consider how linear delay diversity impacts the symbol stream at the MA transmitter device 102 and the SISO receiver device 112. FIG. 4A is a block diagram that shows partial packet structures 200a and 200b, each having three symbols 300 (designated as symbol 1, symbol 2, and symbol 3) comprising guard intervals 302. Packet structure 200b is a delayed version of packet structure 200a. Packet structure 200b is delayed (e.g., by delay element 108) a time delay duration designated as tD.
With continued reference to FIGS. 1-4A, FIG. 4B is a block diagram that illustrates how the SISO receiver device 112 handles the received packet structures 200a and 200b shown in FIG. 4A. That is, FIG. 4B illustrates a single received signal (comprising the addition of the two transmit signals corresponding to packet structures 200a and 200b) output from the single receive antenna 110. The SISO receiver device 112 determines the guard interval and symbol location (and FFT placement) through an acquisition process that includes a correlation of the known pattern corresponding to the LTS (e.g., section C of the OFDM packet 200 (FIG. 2)). Legacy receivers may perform a channel impulse response (CIR) estimate using the LTS. The LTS is designed to elicit an impulse response when correlated with multi-path, thus resulting in a multi-path profile. In the absence of a time delay (or in the absence of multi-path), the response to a received packet is an impulsive correlation (which would be shown as a single impulse 402a in the guard interval 302), enabling proper timing or synchronization of the received signal and proper placement of the FFT. With the time delay (tD) (or with multi-path), two impulsive responses (402a and 402b) are detected. The FFT is placed after the detection of the second impulse response 402b. Thus, FFT placement is accomplished by using the impulse response(s) to center the guard interval 302 on the transients so that inter-symbol interference ISI is minimized.
Linear delay diversity results in true delays that exhibits a behavior from a receiver perspective that is indistinguishable from true multi-path (e.g., the situation where the signal bounces off an object and arrives at the SISO receiver device 112 with a time delay due to the greater path distance traveled). Further, with linear delay diversity, complete destructive interference is avoided, but at the expense of the consumption of the guard intervals 302.
Another technique used to make MA transmitters compatible with legacy receiver devices is referred to as cyclic delay diversity. With cyclic delay diversity, each tone or subcarrier is cyclically shifted (e.g., in the baseband processing portion of the MA transmitter device 102, FIG. 1) by a defined amount of samples, resulting in a circular shift of all of the samples involved (e.g., 64 samples plus 16 samples of the data and guard interval, respectively). FIG. 5 is a schematic diagram that illustrates 1-sample cyclic delay diversity. The first tier 502 represents 80 total samples comprising 64 samples of symbol data 508 and 16 samples that are used for a guard interval 510. The second tier 504 is simply an extension of the first tier, showing the guard interval 510 corresponding to samples 510 adjacent to the next set of samples. This next set of samples comprises 64 samples of symbol data 512 and 16 samples corresponding to the guard interval 514a. The third tier 506 represents a 1-sample cyclic delay shift. As shown, a delay of 1 sample results in the guard interval 514b preceding the 80 sample symbol in the third tier 506, starting with sample 4/7, as indicated by line 516. All of the other samples are likewise shifted as represented by the dotted arrow-head lines 518.
From a receiver standpoint, the guard intervals 510, 514b remain aligned, as represented by the dashed line 520. In other words, with cyclic delay diversity, guard intervals are not consumed as they are in linear delay diversity. The samples used for each guard interval are different after a cyclic shift, but the guard intervals still remain as 16 samples. However, one problem with cyclic delay diversity involves the acquisition process described earlier. When correlation occurs at the SISO receiver device 112 (FIG. 1), the sample shifting is detected, such shifting being indistinguishable from the sample shifting that occurs in linear delay diversity. In other words, the impulse response appears to shift directly with the cyclic shift. However, the true location of the guard interval does not change because the MA transmitter device 102 (FIG. 1) is using cyclic shifts. This can spoof the SISO receiver device 112 such that the FFT placement is erroneous. That is, the SISO receiver device 112 may shift the FFT over in a manner that spills over to the subsequent guard interval.